In recent years, concern about environmental problems has been increasing. Under such circumstances, in order to reduce carbon dioxide that causes global warming, and as an effective material alternative to petroleum that is an exhaustible resource, biomass-based resin, particularly, polylactic acid, has received attention. Polylactic acid has a relatively high melting point (150 to 180° C.), and it has strength comparable to that of a polystyrene. Thus, it is greatly expected that such polylactic acid will become widely used. However, polylactic acid is more expensive than petroleum-based resins, and it has no mechanical properties other than environmental compatibility, which are superior to those of petroleum-based resins. Due to such problems, polylactic acid has not yet become widespread. For polylactic acid to be used in a variety of products, it is important to add new functions to the polylactic acid to enhance the added value thereof.
A new function may be a shape-memory property that is an intelligent function. The shape-memory property is a property, in which a material can be deformed at a predetermined temperature, and such a desirably deformed shape can be fixed by cooling it to room temperature, and recovered to its original shape by heating it again. As materials having such shape-memory property, an shape-memory alloy material and a shape-memory resin have been conventionally known. Shape-memory alloys find use in pipe joints, a straightening teeth, and the like, whereas shape-memory resins in thermal contraction tubes and laminate materials, fastening pins, and medical equipments such as plaster casts. Unlike the shape-memory alloy, the shape-memory resin has the following merits. The resin can be processed into a complicated shape, has a high shape-recovery efficiency, light weight, readily colorable, and low cost. Because of these merits, the shape-memory resin is expected to enlarge the application fields.
The shape-memory resin is characteristically constituted of a reversible phase, which is composed of a non-cross-linked portion and flowable at a predetermined temperature or more (the Tg or melting point in the reversible phase), and a frozen phase, which is composed of a physical or chemical bonded site (cross-linking point).
As shown in FIG. 1, the shape-memory mechanism of a molded product using the shape-memory resin includes 3 steps, namely, memorizing a shape, deforming a molded product, and recovering the memorized shape, as described below.
1. Processed by Molding (Original)
When a shape-memory resin is processed by being heated, being melted, and being solidified, an initial shape (original shape) consisting of a frozen phase and a reversible phase (rigid state) (shown in FIG. 1(a), and a partially magnified view (b) of FIG. 1) is memorized.
2. Deformation of Molded Product
The molded product can be arbitrarily deformed at a temperature, at which only the reversible phase is melted but the frozen phase is not melted, that is, not less than the Tg or melting point of the reversible phase, thereby converting the reversible phase into a soft state (Stage (c) of FIG. 1); followed by applying external force to the molded product while maintaining its state (Stage (d) of FIG. 1). When the molded product thus deformed is cooled to a temperature of Tg or melting point of the reversible phase or lower, the reversible phase is also completely solidified, thereby fixing the deformed shape state (Stage (e) of FIG. 1).
3. Recovery of Memorized Shape
In the molded product arbitrarily deformed, the deformed state of the shape is maintained by the reversible phase forcibly fixed in the meantime. Therefore, when the temperature of the deformed product reached a temperature at which the reversible phase alone softens, the resin exhibits elasticity (rubber-like property) and comes to a stable state. In this way, its original shape is recovered (Stage (c) of FIG. 1). The initial state of the molded product shown in Stage (b) of FIG. 1 is brought back by further cooling to not more than Tg or the melting point.
A shape-memory resin used to produce the above-mentioned molded product can be classified into a thermosetting type and a thermoplastic type, depending on the situation of a frozen phase. In terms of shape-memory performance, the thermosetting type is superior to the thermoplastic type (Non-Patent Document 1). The thermosetting type shape-memory resin has the following advantage. The frozen phase of the thermosetting type shape-memory resin is composed of a covalently cross-linked structure. The resin is highly effective in preventing the fluidization of a resin, excellent in shape-memory and dimensional stability, and recovers the original shape at high speed. On the other hand, the binding force of the frozen phase of the thermoplastic type shape-memory resin is weaker than that of the thermosetting type shape-memory resin having covalent cross-linking, since the frozen phase of the thermoplastic type shape-memory resin is composed of physical cross linked structure for example crystalline part, glass-state region of a polymer, the entanglement of polymers, or a metal crosslink. Thereby, the thermoplastic type shape-memory resin is inferior in shape recovering property to the thermosetting type shape-memory resin.
By the way, there have been reported several cases in which a shape-memory property is imparted by three-dimensionally cross-linking polylactic acid via a chemical bonding. For example, there has been reported a shape-memory resin in which polylactic acid is cross-linked by irradiation with an active energy ray (Patent Document 1). However, since such crosslink by irradiation with an active energy ray does not adopt a perfect three-dimensional structure, the shape-memory performance of the concerned shape-memory resin is lower than that of a thermosetting resin. Moreover, since it requires high costs for equipments and a shaping process has a certain limit, it is difficult to produce a large product and the like.
The present inventors had already developed a thermo-reversible shape-memory resin, into the cross-linking site of which a covalently-bound thermo-reversible reaction is introduced, as a new material having the advantages of both a thermosetting resin and a thermoplastic resin (Patent Document 2). The thermo-reversible reaction is a reaction in which a bond is cleaved at a predetermined temperature and it is then rebound when cooled. This thermo-reversible reaction is described in Non-Patent Document 2. A shape-memory resin cross-linked by such thermo-reversible reaction has a frozen phase as a thermo-reversible cross-linking site and a reversible phase as a resin, and because of the two phases, the resin has a shape-memory property. Specifically, since the shape-memory resin is three-dimensionally cross-linked via a covalent bonding in a practical temperature region and it functions as a thermosetting resin, this resin has excellent shape recovering and dimensional stability, and a reduction in the shape recovery rate due to repeated deformation is suppressed. Since this resin functions as a thermoplastic resin when it is heated to a temperature at which the bond of the thermo-reversible cross-linking site is dissociated, the resin is melted and is remolded to another shape, namely, it can be recycled. Furthermore, when the recycled resin is cooled, the cross-linking site is associated, and the resin returns to a thermosetting resin. Thus, excellent shape-memory ability can be reproduced. That is to say, this resin can be a shape-memory resin having both advantages such as excellent shape-memory performance and recyclability. The present inventors had developed a thermo-reversible shape-memory resin with further improved mechanical strength by introducing a chain structure capable of relaxing the inner strain of resin (Patent Document 3).
Polylactic acid having such cross-linking site by a thermo-reversible reaction has good mechanical strength, however it does not have sufficient toughness. If the toughness of the polylactic acid can be improved, it can be applied to produce high durable products, and the like.
As shape-memory polylactic acid having high toughness, there has been known polylactic acid cross-linked with a flexible segment, for example (Patent Document 4). However, the strength of such polylactic acid is not sufficient to be applied to durable products. Hence, it has been desired to develop a resin preferably used for high durable products.    Patent Document 1: JP10-147720A    Patent Document 2: WO2005/056642    Patent Document 3: JP2006-331921B    Patent Document 4: JP2002-504585A    Non-Patent Document 1: Masao KARAUSHI, “Keijyo Kioku Polymer no Zairyo Kaihatsu (Development of Materials for Shape Memory Polymers)” CMC Publishing CO., LTD., pp. 30-43, 1989    Non-Patent Document 2: Engle et al., J. Macromol. Sci. Re. Macromol. Chem. Phys., Vol. C33, No. 3, pp. 239-257, 1993